Which of these animals does NOT belong in class Arachnida?
A tick
B scorpion
C mite
D spider
E crayfish
The correct answer and explanation is:
Correct Answer: E. Crayfish
Crayfish do not belong in the class Arachnida. Instead, they are classified under the class Crustacea. Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata, which includes spiders, scorpions, ticks, and mites. These animals typically have two main body segments: the cephalothorax and the abdomen. They also usually have eight legs and lack antennae.
Crayfish, on the other hand, are aquatic animals found in freshwater environments. They are members of the class Crustacea, which includes other familiar animals like crabs, lobsters, and shrimp. Crustaceans usually have a segmented body with a hard exoskeleton, two pairs of antennae, and multiple pairs of legs. Crayfish typically have five pairs of walking legs, the first of which are often enlarged into claws.
Let us compare:
- Tick: A small arachnid that is an ectoparasite, feeding on the blood of mammals, birds, and sometimes reptiles.
- Scorpion: An arachnid known for its segmented tail with a stinger and pincers on the front legs.
- Mite: A very small arachnid, often microscopic, found in soil, water, and even on animals.
- Spider: The most well-known arachnid, characterized by eight legs and the ability to produce silk from spinnerets.
- Crayfish: Not an arachnid. It lives in water, breathes through gills, and is more closely related to crabs and lobsters.
All arachnids are primarily terrestrial, while crayfish are aquatic. This ecological difference, along with distinct anatomical features, places crayfish in a separate class altogether. Thus, crayfish do not belong in the class Arachnida.